Excavation of solids such as rock, coal, stone, ores, and the like is required in a variety of circumstances. These include in the mining industry for recovering ore, quarries, recovery of coal, sinking of bore holes, excavation of tunnels and the like for sewerage, conduit lines, vehicles, and cutting of trenches, channels and the like.
It is known to excavate solids using basting techniques. However, blasting techniques are less suitable for excavation of tunnels, trenches or channels. Blasting also requires strict safety precautions.
Another technique to excavate solids is by using cutting or grinding apparatus. Such apparatus are provided with one or more cutting discs which are provided with peripheral teeth (or picks). The discs are forced against the solid to be cut at extremely high forces and cause the solid to be cut away in small pieces.
A disadvantage with this type of apparatus is that the cutting process is extremely energy intensive and extremely energy inefficient. The technique releases large amounts of heat and requires large equipment to provide the necessary power to rotate the cutting discs and to force the cutting discs against a rock face.
Examples of such apparatus are found in the Wohlmeyer U.S. Pat. Nos. 2,758,825, 3,297,101 and 3,379,024.
One disadvantage with excavating rock in a purely cutting process, is the enormous wear and tear placed on the cutting disc and its cutting teeth. Thus, a high degree of maintenance is required on the cutter discs.
The apparatus is also of a large size to accommodate the equipment necessary to power and hold the cutter discs. Thus, the apparatus is expensive to use and maintain and is unsuitable for relatively small-scale excavations. These known apparatus are currently used in tunnelling.
In order to improve the effectiveness of solid removal, it has been known to use a principle called "cut and break". In this process, an apparatus initially cuts a groove in the solid with a cutting disc and then inserts a wedge into the cut groove in a separate action. The wedge causes the rock or other solid to break away from the groove. Thus, excavation of the solid is due to a cutting action and a separate breaking action.
A known apparatus using this cut and break technique is known as a "McKinlay Entry Driver" which appears to have been used in the U.S.A. in about 1918.
The apparatus was limited in its use to cutting holes for tunnels and the like and was unable to cut trenches.
This apparatus consisted of a horizontal shaft to which an arm was mounted at right angles. The arm extended to each side of the shaft. To the arm were fixed a number of cutting plates which extended at right angles to the arm. Upon rotation of the shaft, the arm was rotated and therefore the cutting plates were rotated in a circular motion. When the machine was forced against a coal face, it caused annular grooves to be cut into the coal face. On a secondary arm was fixed a wedge which was exactly positioned such that upon rotation of the shaft, the wedge was caused to enter into one of the grooves formed by the cutting plate therefore causing the coal to burst apart.
This type of apparatus still suffered from a number of disadvantages including its size, complexity and expense.
Another disadvantage was the requirement to correctly position the wedge on the secondary arm to ensure that it was caused to enter into a cut groove and did not scrape against the coal face. If a particular cutting plate was damaged and therefore resulted in a different type of groove being cut, this could result in the wedge not properly entering into the groove. Furthermore, if a cutting plate was replaced, it was often necessary to ensure that the wedge was repositioned correctly.
Another disadvantage with the wedge (also called a "wedging wheel" or "bursting wheel") was the possibility of it becoming trapped in the groove. Should this occur, and if the rock was extremely hard or did not burst, the result was that either the wedge would be destroyed or deformed or that the motor driving the shaft would burn out.
Should the wedge be deformed or destroyed, it was necessary to shut down the apparatus for replacement and re-positioning of the wedge.
Due to the difficulties with the wedges, the more modern cutting machines dispensed with them altogether.